1. Field of the Invention
The present invention relates generally to telephone subscriber line interface circuits (SLIC's) for telecommunication transmission, data communication and telephone switching system applications. More specifically, the present invention relates to a SLIC circuit that is cost efficient, manufactuable in a small size, and capable of providing direct resistive line feed impedance by means of impedance synthesis, while also using a dc blocked transformer, resulting in improved line circuit transient protection, including protection of the line circuit solid state components from lightning.
2. Description of the Prior Art
As is well known, telephone line circuits are designed to provide the BORSHT functions (battery feed, overvoltage protection, ringing, signalling, hybrid and timing). Known methods of interfacing telephone subscriber lines to the local or central office switching equipment include transformer coupling with a split primary winding and some form of balanced resistors for current feeding to all solid state direct coupled circuits. The more stringent the circuit specifications, the more costly and complex such SLIC circuits have become. For example, for central office and integrated digital loop carrier (IDLC) applications, known line circuits must provide good longitudinal balance, longitudinal current capability for peak currents that can cause instantaneous reversal of the loop current direction, while also maintaining both transmission quality and the capability to transmit and receive voice band signals in the "on-hook" state. While these requirements are by way of example only, they illustrate that line circuits must be rugged and able to survive lightning induced voltage surges up to 1000 volts.
The Bell Communications Research specifications, in pertinent part, for IDLC may be found with reference to Bellcore Technical Reference TR-TSY-000 303, Issue 1, Sept. 1986, Revision 1, Dec. 1987. The known line circuit technique using a split primary transformer in conjunction with balanced (equal value) resistors which provide current feed to the subscriber line has sufficient limiting for short loops. The resistors and battery feed voltage source e.g. the exchange battery, are bypassed by a mid-point capacitor, which allows coupling the secondary side voice signal to the subscriber line with low loss. The above described line circuit configuration is simple, rugged and the signal processing electronics are transformer isolated against common mode voltages on the primary side. Balanced (equal) resistors allow bilateral current flow, e.g. there is no intrinsic limit to the level of 60 H.sub.Z longitudinal currents that may be present on the subscriber line without degrading the voice signal. The above described known line circuits have the disadvantage of requiring bulky and heavy transformers. In view of the limitations of magnetic materials, it is not possible to build a small transformer able to carry substantial dc current (the line current). This is because the inductance of the transfomer rapidly collapses, e.g. flux saturation, in any form of high permeability, e.g. ungapped, core carrying dc current. Sufficient inductance necessary for voice transmission can be achieved in a small and inexpensive transformer if the transformer is dc blocked, e.g. only ac voice signals generate magnetic flux.
Many line circuits of the prior art use all solid state direct coupling techniques. An example of such an all solid-state line circuit is described in the IEEE Journal of Solid-State Circuits, Vol. SC-16, No. 4, Aug. 1981. Direct coupling to the line results in a higher risk of circuit failure from lightning induced transients, hence costly protective circuits have been designed to reduce this risk. Line circuits of the known prior art having small size and using all solid-state technology, e.g. integrated circuit technology, has not, to date, been achievable on a cost equivalent basis with the transformer coupled circuitry, also of the prior art.
The concepts of ac and dc impedance synthesis, i.e. the use of voltage or current feedback techniques to synthesize the ac transmission termination characteristics of a SLIC are known. For example; in U.S. Pat. No. 4,317,963, a SLIC using impedance synthesis for loop current regulation; in U.S. Pat. No. 4,387,273, wherein a solid state line circuit provides for independent control of the transverse and longitudinal impedances and for the adjustment of the equivalent termination impedance, upwardly or downwardly from a lower resistance value; in U.S. Pat. No. 4,254,305 wherein current limiting a line feed circuit is described; and in U.S. Pat. No. 4,351,060 wherein digitally synthesizing matching line terminating impedances using digital filter techniques is described. The concept of fluxbucking a line transformer in a line circuit having electronic equivalent line terminating impedances is also known in U.S. Pat. No. 4,241,239.
In accordance with the present invention, an improved SLIC circuit has been devised, which incorporates the positive aspects of magnetic and solid-state technology, e.g. the dc blocked transformer for small size and low cost and solid state circuitry for low voltage signal processing.